In order to maintain healthy growth, plants must extract a variety of elements from the soil in which they grow. These elements include phosphorus and the so-called micro-nutrients (e.g., copper, iron, zinc, etc.), but many soils are deficient in such elements or they contain them only in forms which cannot be readily taken up by plants (it is generally believed that essential elements cannot be readily taken up by plants unless they are present in dissolved form in the soil).
To counteract such deficiencies, sources of the deficient elements are commonly applied to soils in order to improve growth rates and yields obtained from crop plants. For example, phosphates are often added to soil to counteract a lack of available phosphorus. Phosphate added to the soil as a commercial fertilizer (e.g., mono-ammonium phosphate, triple-super-phosphate, etc.) is readily plant available, but is rapidly converted in soil to relatively unavailable forms. It has been estimated that only 10 to 30% of phosphate fertilizer is used by the plant in the year it is applied, and one-third to one-half of the phosphate fertilizer applied may never be recovered by the plant.
Attempts have been made in the past to use microorganisms to improve the availability of essential elements in soil systems. In particular species of the fungus Penicillium has been used for this purpose.
For example, U.S. Pat. No. 5,026,417 discloses isolated strains of P. bilaii which are capable of improving the uptake of phosphorus by plants when applied to the soil.
U.S. Pat. App. Pub. No. 2010/0099560 discloses method of enhancing growth conditions for plants by growing the plants in soil containing, in proximity to the plant roots, both a phosphorus source and at least two strains of the fungus Penicillium. 
U.S. Pat. No. 5,484,464 Methods and compositions for increasing the availability of soluble phosphate and fixed nitrogen for legume: Rhizobium symbioses involving co-inoculating legume seeds with a phosphate-solubilizing soil fungus, Penicillium bilaii, and Rhizobium spp. prior to planting.
There is, however, still a need for systems for improving growth conditions for plants, particularly by increasing the levels of available phosphorus in soil systems.